If the main breaker in the main panel is 100A.
And you have a subpanel with only light load, say 20A to 30A each breaker totally at most 70A simultaneous use of all breakers.
Do you need to use wire gauge sized for 100A (AWG 2 instead of just AWG 4)? I'm aware you can use the 90 degree table.
Rational is if the subpanel feeder wire is small for the main panel main breaker, it would heat up? But what if the load won't reach that amperage. Then you can use smaller feeder wire just enough for the main breaker in main panel to trip if there is a short, and not deal with overloading which won't happen in the subpanel because the load is sized for only 70A and AWG 4?
Or does the rule say the subpanel feeder wire need to match the main panel main breaker no matter how light the load in subpanel so need to use AWG 2?